1. Field of the Invention
The invention relates to a method of manufacturing a solid electrolytic capacitor, and more particularly to a method of manufacturing a solid electrolytic capacitor including the conducting polymer compound as solid electrolyte.
2. Description of the Related Art
With development of science technology, electrical devices are required to be down-sized much more and have greater reliability. With respect to a capacitor, it is also desired to develop a solid electrolytic capacitor having a performance sufficiently covering a high frequency band, and also having a reliability and a large capacity. In order to manufacture such a capacitor, much of research and development has been conducted.
In general, a solid electrolytic capacitor includes a first electrode or an anode composed of porous compact of valve action metal such as tantalum (Ta) and aluminum (Al), an oxide film derived from the valve action metal and serving as a dielectric substance, and a second electrode or a cathode partially composed of solid electrolyte such as manganese dioxide (MnO.sub.2) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex salt. The solid electrolyte is required to have functions for electrically connecting an entire dielectric substance disposed in the porous compact to leads of the electrodes, and also for repairing electrical short-circuit derived from defects of the oxide film. Accordingly, it is not allowed to use metal having high electrical conductivity, but having no function for repairing electrical short-circuit, as a solid electrolyte. For instance, manganese dioxide (MnO.sub.2), which can be converted to insulative material due to heat generated by short-circuit current, has been often used as a solid electrolyte.
However, a solid electrolytic capacitor having manganese dioxide (MnO.sub.2) as a part of an electrode cannot have a sufficiently high electrical conductivity, and hence the impedance in high frequency band cannot be reduced. In addition, a solid electrolytic capacitor having TCNQ complex salt as a part of an electrode can have only a poor thermal resistance, because TCNQ complex salt is easy to be thermally decomposed.
Recently, new materials have been developed in the field of high polymer. For instance, there has been developed a conducting polymer including conjugate high polymer such as polyacetylene, poly-p-phenylene, polypyrrole and polyaniline to which is added a dopant or an electron donative and/or electron attractive compound. Among these conducting polymers, polypyrrole is widely used as a solid electrolyte of a solid electrolytic capacitor, because polypyrrole is superior to other conducting polymers in terms of electrical conductivity and stability. For instance, Japanese Unexamined Patent Public Disclosure No. 63-158829 has suggested the use of polypyrrole for a solid electrolyte of a solid electrolytic capacitor.
However, while a conducting polymer compound layer is being formed, a conducting polymer compound layer often adheres to an anode lead, because monomer solution used for forming the conducting polymer compound layer has the low viscosity. As a result, when an external anode lead terminal is welded to an anode lead, a conducting polymer compound layer serving as a cathode is in contact with the external anode lead terminal. This poses a problem that it is impossible to ensure electrical insulation between the conducting polymer compound layer and the external anode lead terminal.
On the other hand, there has been known in the art the method in which an insulative layer is formed on an anode lead to thereby prevent that an anode lead is covered with a solid electrolytic layer. For instance, Japanese Unexamined Patent Public Disclosure No. 60-948 has suggested such a method. Although the method suggested in the Disclosure is directed to solving a short-circuit problem in a capacitor having manganese dioxide (MnO.sub.2) as solid electrolyte, it is possible to apply the method to a capacitor using conducting polymer compound therein. In accordance with the method, it is possible to prevent conducting polymer compound from being formed on an anode lead. However, though the conducting polymer compound is not formed on an anode lead, the conducting polymer compound is formed on an insulative layer. As a result, there still remains a possibility that a conducting polymer compound layer is in contact with an external anode lead terminal when a capacitor is under thermal expansion or shrinkage.